1. Field of the Invention
This invention relates to integrated circuit technology and, more specifically, to antifuse devices which provide an electrically programmable connection between two unconnected points of a circuit. More specifically, the present invention relates to a method and apparatus for programming antifuse elements by applying a combination of AC and DC electric fields.
2. Related Art
This application relates to co-pending patent application Ser. No. 08/249,870 filed herewith, hereby incorporated by reference, and assigned to Symetrix Corporation.
3. Brief Description of the Prior Art
It is known to provide programmable integrated circuits which permit a user to customize the application of the circuit by programming ("blowing") electronic interconnections within the circuit to fulfill a specific application need. Integrated circuits such as Programmable Array Logic (PALs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), and Programmable Read-Only Memories (PROM's) are typical of such programmable devices. These devices provide arrays of programmable elements which a user may elect to connect or disconnect by electrical means. "Fusible" devices are manufactured with each element initially connected and are programmed by applying sufficient current to an element to "blow" the fuse thereby disconnecting the element. "Antifuse" devices are manufactured with each element initially unconnected and are programmed by applying an electrical field across an element sufficient to cause a breakdown of an insulating material within an element. The breakdown of the insulating layer forms a conductive filament connecting the two previously unconnected points of the element.
A large class of such devices are one-time programmable (OTP) devices in that each element may be programmed only once to meet the requirements of a particular application. Once programmed, the element cannot be changed again. Most antifuse devices use a dielectric layer of material between the unconnected contacts to provide the electrical insulation between the contacts. In an electric field of sufficient strength, the dielectric material heats to the point of breakdown and forms a conductive filament through an aperture in the dielectric material. This conductive filament connects the two previously unconnected contacts of the programmable antifuse element. Prior designs have taught the use of exclusively DC electric fields to program an antifuse element.
Basire et al. in U.S. Pat. No. 4,488,262 (issued Dec. 11, 1984) teach the design of a PROM device implemented with an array of antifuse elements. Basire et al. disclose a device embodying a two dimensional array of antifuse elements with intersecting row and column electrodes disposed across the insulative layer of each antifuse element. An electric field is controllably applied across each antifuse element to be programmed. The field is applied to the row electrode and column electrode which intersect at the antifuse element to be programmed. A sufficiently large electric field breaks down the insulative layer between the two intersecting electrodes to form a conductive filament through the insulative layer.
It is desirable to use materials for the insulating layer which have high resistivity in the unprogrammed state and yet breakdown to form a conductive filament of low resistivity. However, such materials have presented problems in prior designs. Although it is desirable to maintain a high resistivity in the unprogrammed state, a high dielectric constant of the insulating material in the unprogrammed state requires a correspondingly high DC electric field amplitude to force the breakdown of the insulating material. When the required field amplitude rises too high for practical applications, some prior designs have taught the use of dopants applied to the electrodes or the insulating layer to improve conductivity. Hamdy et al. teach the use of arsenic dopants applied to one or both electrodes in U.S. Pat. No. 4,889,205 (issued Feb. 6, 1990). The arsenic is intended to reduce the resistivity of the conductive filament formed by the breakdown of the insulative layer. Similarly, Lee teaches in U.S. Pat. No. 5,250,459 (issued Oct. 5, 1993) the use of antimony as a dopant to be applied to one or both electrodes. The antimony flows into the conductive filament when formed by the breakdown of the insulative layer. However, these approaches typically alter both the programmed and unprogrammed resistivity.
4. Solution to the Problem
The present invention teaches the use of a combination of AC and DC electric fields to break down the dielectric layer to form a conductive filament between two previously unconnected electrodes within the element. That is, an AC electric field is first applied across the antifuse element, followed by a DC electric field which causes the dielectric material to break down and form a conductive filament connecting the two previously unconnected electrodes. Materials such as ferroelectrics and perovskites exhibit the property that their dielectric constant rises significantly in the presence of a DC electric field. The high dielectric constant of these materials increases the power dissipated through dielectric heating within the material in response to an applied AC electric field. The combined effects of the AC and DC electric fields breaks down the ferroelectric material more easily than could either field alone.
The application of ferroelectric and other similar materials in the dielectric layer of antifuse elements is recited in co-pending U.S. patent application Ser. No. 08/249,870.
The present invention is applicable to dielectric layer materials in which the dielectric constant increases with the application of a DC electric field. The dielectric heating effect of the AC electric field is enhanced by the increase in the dielectric constant caused by the simultaneous application of a DC electric field. In materials with electric dipole properties, such as ferroelectrics, the application of a DC electric field increases the dielectric constant so that an AC electric field will cause sufficient dielectric heating to breakdown the material.